Grease (2) 9'' x 5'' inch loaf pans with butter and set them aside. Cook the potato (you'll need about 3 potatoes or enough to make 5oog) in a microwave on HIGH until soft and it squeezes easily, using the designated baked potato setting, or up to 15 minutes. Peel and pass the warm potato through a potato ricer or food mill into a large bowl. (You should have 500g pureed potato.) Set aside to cool.
Alternatively, you can place potatoes in a saucepan, cover them with water to a depth of 1 inch, bring them to a boil, lower the heat to a simmer, and cook until the potatoes are tender when pierced with a knife. Drain potatoes and follow as stated above.
In the bowl of a stand mixer, combine the yeast, 125g (1 cup) of flour, and 8 oz (1 cup) of warm water (about 110 to 115 F). Let the mixture sit for 10 to 15 minutes until it becomes frothy.
In another bowl, combine the remaining 875 g ( about 7 cups) flour, salt, and sugar and set it aside. Add the oil, eggs, and mashed potatoes to the stand mixer bowl with the yeast mixture. Using the dough hook attachment, mix on low speed until the ingredients are well combined.
Gradually add the flour mixture to the stand mixer bowl in 3 to 4 increments, mixing on low speed after each addition until the dough comes together and is smooth and elastic; this should take about 8 minutes. Add a tablespoon or two of warm milk if the dough is too dry.
Form the dough into a ball and place it in a lightly oiled large bowl; turn to coat the entire dough with oil, then fold it over itself a few times to form into a ball. Cover with a clean kitchen towel and let it rise in a warm, draft-free area for about 1 to 2 hours or until it has doubled in size.
Once the dough has risen, gently punch it down to release any air bubbles. Divide the dough into two equal parts. Shape each half of the dough into a loaf and place each loaf in the prepared pans, making sure to tuck the edges of the dough underneath to create a smooth surface on top. Cover and let rise until dough is just above the tops of pans, 45 minutes to 1 hour.
Preheat the oven to 350F. Bake the bread loaves in the preheated oven for 35-40 minutes, or until the loaves sound hollow when tapped, the tops are brown, and the internal temperatures are 200 degrees. Remove the bread from the oven and let them cool in the pans for 5 minutes before transferring them to a wire rack to cool completely.
👀👉Note: It's important to make sure that your leftover mashed potatoes are at room temperature before adding them to the dough. This will allow the mashed potatoes to mix more easily with the other ingredients, resulting in a smoother and more uniform dough. Additionally, using room-temperature mashed potatoes can help ensure that the dough rises properly.